1. Field of the Invention
The present invention relates to an image processing method and apparatus applicable to an image output device for forming, on a base such as a sheet or the like, a color image of process color materials {C (cyan), M (magenta), Y (yellow), and K (black)} and a base color material, e.g., W (white), whose colors are determined by the total amount of color materials that are applied per pixel, and a CPU-readable recording medium storing therein a program for carrying out such an image processing method.
2. Description of the Related Art
Electrophotographic printers which employ toners as color materials, and image output devices (image forming devices) such as ink jet printers or the like and printing presses which employ inks as color materials operate to form an image on a sheet (a support or a recording medium) of paper (printing paper) by applying color materials per pixel.
The colors of the image formed on the sheet are affected by the background color of the sheet. There has been proposed the technology of an ink jet printer for forming a desired color image on a yellowish white sheet such as a recycled sheet of paper, a postal card, or the like while preventing the color image from being affected by the yellowish white which is the background color of the sheet (see Japanese Laid-Open Patent Publication No. 2002-038063).
According to the proposed technology, a white ink is applied from an ink set to a yellowish white sheet and then dried to turn the background color of the yellowish white sheet into white. Thereafter, process inks including yellow, magenta, light magenta, cyan, light cyan, black, etc. are applied from the ink set additively to form a color image on the sheet.
In reality, the total amount of color materials that can be applied to or added on the sheet per pixel is fixed for each image output device. The range of C, M, Y, K values that can be adopted in an image output device depends on the total amount of color materials used in the image output device.
For example, as shown in FIG. 5A of the accompanying drawings, if each of the four color materials C, M, Y, K can be applied up to 100% on a sheet S per unit area, then the upper limit of the total amount of color materials is 400% {(C, M, Y, K)=(100, 100, 100, 100)}.
If the total amount of color materials has an upper limit of 300%, then, as shown in FIG. 5B, each of the four color materials C, M, Y, K can be applied up to 75% on a sheet S. In this case, although not shown, if the color material C is applied by 100%, then the sum of the remaining three color materials M, Y, K is limited to 200%.
If color materials are applied in excess of a total amount of 100% and tend to droop, as shown in FIG. 5C of the accompanying drawings, then the total amount of color materials is limited to an upper limit of 100%.
In the description of the present specification, the amount of a color material is expressed in a range from 0% to 100% which corresponds to a gradation range of 8-bit image data from a gradation level 0 to a gradation level 255. Specifically, the maximum gradation level 255 corresponds to 100%, the minimum gradation level 0 corresponds to 0%, and the intermediate gradation level 127 corresponds to 50%. The percentage % may be considered to be halftone dot %. If the image data represent a halftone image, then each halftone dot cell comprises a plurality of pixels.
When a white ink is applied to the entire surface of an image forming area on a sheet as disclosed in Japanese Laid-Open Patent Publication No. 2002-038063, since the total amount of inks that can be applied on the sheet is finite in reality, the inks can be applied only in a certain range, tending to result in image formation problems that inks droop and cannot be fixed in place. Nevertheless, Japanese Laid-Open Patent Publication No. 2002-038063 does not refer to anything addressed to the above problems.